1. Field of the Invention
The present invention relates to friction welding machines.
2. Description of the Prior Art
Friction welding is generally defined as a solid state bonding process in which one metal workpiece is rotated at relatively high controlled speed against a stationary workpiece. The workpiece surfaces are heated by frictional contact to high temperatures and forged together to produce the bond, the bond being completed within seconds after frictional contact is made. Friction welding is advantageous not only because of the short bonding cycle but also because only a narrow heat affected zone is produced in the base metal adjacent the weld.
A type of friction welding known as inertia bonding is widely used to join metal workpieces together. In a typical inertia bonding device, such as that illustrated in U.S. Pat. No. 3,273,233, the workpiece to be rotated is fixtured in a headstock assembly while the nonrotating workpiece is fixtured in a tailstock assembly, each assembly being separately bolted or otherwise secured to the bed of a suitable machine. The headstock assembly generally includes a spindle having a chuck at one end and an inertial mass, such as a flywheel, in proximity thereto. At the other end is located a drive mechanism or other means for rotating the spindle. The tailstock assembly is somewhat simpler and includes a slidable spindle keyed against rotation, the spindle having a chuck at one end. The headstock and tailstock spindles are mounted on separate bearing surfaces to position the chucks in opposed relation so that the workpieces are held in predetermined alignment. However, shimming is often required to obtain the desired workpiece alignment, especially if close concentricity tolerances are called for.
In operation, a suitable motor rotates the headstock spindle to a predetermined angular velocity, the motor is disengaged and the tailstock spindle is advanced toward the headstock assembly to engage the nonrotating workpiece with the rotating workpiece. Sufficient axial pressure is exerted across the interface to bring the workpiece surfaces to the bonding temperature. During frictional engagement, three stages in the bonding process are discernable. In the first, localized seizure and rupture of the seized areas occurs at the interface, as evidenced by a rise in the measured torque. The angular velocity of the flywheel decreases as a result of the conversion of kinetic energy to heat at the interface. At the beginning of the second stage, the temperature and pressure needed for bonding are present at the interface but the flywheel velocity is still above the critical value at which bonding can occur. As the flywheel velocity decreases further and approaches the critical value, there is a transition from the second to the third stage during which the measured torque rapidly increases to an extremely high value, indicating that a bond has formed across the interface. In the third stage, after bond formation, extensive plastic working of the bond occurs as a result of the extremely high torque present. Plastic working of the bond quickly dissipates the remaining kinetic energy of the flywheel and it finally comes to rest, the bonding process being completed.
As mentioned, in the prior art bonding devices, the headstock and tailstock assemblies are separately secured to the machine bed. Although these assemblies may be quite massive and be very securely mounted to the machine, the torque developed during and after bond formation is so high that relative radial movement of the assemblies is oftentimes observed. Torques greater than 100,000 pound-feet are common and have been observed to plastically strain the bolts holding the assemblies to the machine. Of course, the amount of torque developed will depend on the size of the workpieces being bonded, larger pieces generating higher torque. Radial movement between the headstock and tailstock assemblies is manifested by misalignment, such as a loss of concentricity, between the joined workpieces which necessitates further corrective machining operations or, in some instances, complete rejection of the bonded part.
Misalignment between the bonded workpieces is further aggravated by the prior art practice of fixturing the workpieces in chucks at the ends of the headstock and tailstock spindles in cantilever fashion. In this arrangement, no means are immediately adjacent the bond interface for supporting the chucks against the extremely high torque generated there.